This invention relates to a multilayer thin film metallic pad structure to be formed on a ceramic substrate for subsequent joining of input/output (I/O) pins. More particularly, this invention relates to a TiO layer for improving the adhesion of the pad structure to the ceramic substrate.
Ceramic substrates, usually and preferably multilayered, are used in the production of electronic substrates and devices. Many different types of structures can be used, and a few of these structures are described below. For example, a multilayered ceramic circuit substrate may comprise patterned metal layers which act as electrical conductors sandwiched between ceramic layers which act as insulators. The substrates may be designed with termination pads for attaching semiconductor chips, connector leads, capacitors, resistors, I/O connector pads, etc. Interconnection between buried conductor levels can be achieved through vias formed by metal paste-filled holes in the individual ceramic layers formed prior to lamination, which, upon sintering, will become a sintered dense metal interconnection of metal-based conductor.
In general, conventional ceramic substrates are formed from ceramic greensheets which are prepared by mixing a ceramic particulate, a thermoplastic polymer binder, plasticizers, and solvents. This composition is spread or cast into ceramic sheets or slips from which the solvents are subsequently volatilized to provide coherent and self-supporting flexible greensheets. After blanking, stacking and laminating, the green sheets are eventually fired at temperatures sufficient to drive off the polymeric binder resin and sinter the ceramic particulates together into a densified substrate.
The electrical conductors used in formation of the electronic substrate may be high melting point metals such as molybdenum and tungsten or a noble metal such as gold. However, it is more desirable to use a conductor having a low electrical resistance and low cost, such as copper and alloys thereof.
With respect to I/O pads, there are inherent thermal stresses experienced in the ceramic substrate due to the thermal coefficient of expansion (TCE) mismatch between the I/O pads and the ceramic substrate. The thermal stresses can lead to cracking of the ceramic substrate or delamination of the I/O pads. This problem has been found to be particularly serious in glass ceramic materials such as those disclosed in Kumar et al. U.S. Pat. No. 4, 301,324, the disclosure of which is incorporated by reference herein.
In an effort to reduce these thermal stresses, Arnold et al. U.S. Pat. No. 4,835,593, the disclosure of which is incorporated by reference herein, have proposed a thin film I/O pad structure consisting of, for example, a titanium adhesion layer, a thick stress-reducing layer of copper or nickel, a barrier layer of titanium and a solder wettable layer of gold.
The use of TiO as an adhesion layer in a low stress thin film I/O pad has so far gone unappreciated in the prior art.
Bhattacharya U.S. Pat. No. 4,514,751 have proposed an adhesion layer of titanium with interstitial oxygen (not TiO) for the contact metallurgy of an integrated circuit device.
It is, therefore, an object of the present invention to have an improved adhesion layer for an I/O pad that is to be formed on a ceramic substrate.
It is another object of the present invention to have a low stress I/O pad that will not cause cracking of the ceramic substrate or result in delamination of the I/O pad.
It is yet another object of the present invention to have an improved adhesion layer made from TiO.
These and other objects of the invention will become apparent after referring to the following description considered in conjunction with the accompanying drawings.